GREEN BAY, Wis. – Jordy Nelson was left wondering Sunday when the Green Bay Packers will get through a preseason unscathed.

"Unfortunately, we're used to it," Nelson, the team's seventh-year wide receiver, said after taking part in a practice with some teammates missing.

The Packers started the final week of the preseason with two big losses. Veteran nose tackle B.J. Raji and first-year starting center JC Tretter, a former Cornell University offensive lineman, are out indefinitely.

Coach Mike McCarthy confirmed after Sunday's practice that Raji suffered a significant biceps injury two days earlier in the Packers' preseason win over the Oakland Raiders. Tretter sustained a significant knee injury in Friday's game.

"JC was injured very early. I don't know if it was on the first series," McCarthy said.

But Tretter kept playing with the rest of the starters until they left the game late in the first half.

"It was bothering him at halftime, then when I got back in here after the game they ended up putting him on crutches," McCarthy said.

The Packers will be breaking in another new starting center with Tretter sidelined for several weeks.

And they won't have much time to get rookie Corey Linsley ready to hold down the important spot in front of quarterback Aaron Rodgers.

Green Bay plays its last preseason game Thursday against the Kansas City Chiefs, then has only a week to prepare for the NFL season opener Sept. 4 at defending Super Bowl champion Seattle.

"This will be my millionth center, so it's not anything new to us," joked Pro Bowl left guard Josh Sitton, a seven-year veteran.

The Packers had been working since their offseason program started in April to train Tretter to be their fourth different starting center going into a season in the past four years.

Tretter, a converted tackle who didn't play as a rookie last season because of a leg injury, had been up to the challenge. He settled into his new position at the start of training camp last month.

After devoting a lot of time to establish continuity on the offensive line with Tretter anchoring the middle of it, the Packers must get Linsley up to speed in a hurry.

"It's just going to take a little extra work by everybody to make sure that he's 100 percent comfortable going out there," right guard T.J. Lang said.

Raji is expected to miss the entire season after he undergoes surgery in the coming days. He watched practice Sunday with his right arm in a sling. When asked whether Raji could be placed on the temporary injured-reserve list, which would allow him to play later in the season, McCarthy replied, "I would say no."

Raji, a former top-10 draft pick, was looking to bounce back this season after re-signing with Green Bay as a free agent on a one-year contract. The Packers will have to compensate for the absence of the 337-pound Raji by turning to a few young prospects, including second-year player Josh Boyd.